Interpretive Summary: Techniques to reliably calibrate computer models are needed before the models can be applied to help solve natural resource problems. The USDA/ARS Root Zone Water Quality Model (RZWQM) is a comprehensive simulation model designed to predict hydrologic and chemical response, including potential for ground-water contamination, of agricultural management systems. RZWQM was calibrated and evaluated at sites in Iowa, Minnesota, Missouri, Nebraska, and Ohio as part of the Management Systems Evaluation Areas (MSEA) research project and at a site near Sterling in Northern Colorado. Following calibration RZWQM predicted values within 20-50 kg ha-1 of measured values for soil nitrogin. Evaluations of the RZWQM plant growth component focused total biomass, yield, N uptake, and nitrogen in the soil profile. Predictions were found to match the observed data in most cases. The crop model is very sensitive to plant N content. Even small errors in simulating nitrogen uptake levels can result in error in estimating yield and total above ground biomass. The model predicted biomass and yield well on irrigated and most dryland systems and simulated crop variables at various positions along the landscape.

Technical Abstract:
Techniques to reliably calibrate computer models are needed before the models can be applied to help solve natural resource problems. The USDA/ARS Root Zone Water Quality Model (RZWQM) is a comprehensive simulation model designed to predict hydrologic and chemical response, including potential for ground-water contamination, of agricultural management systems. RZWQM version 3.2 was calibrated and evaluated at sites in the Management System Evaluation Areas (MSEA) research project. Soil horizon description and a description of the physical and hydraulic properties of the soil were required to initialize the model. Nutrient cycling was calibrated by adjusting coefficients for mineralization, infiltration, and denitrifica- tion. Initial nitrogen pool sizes were estimated using medium to long term computer simulations. Maximum nitrogen uptake rate, plant respiration, specific leaf area, and the effect of age at the time of propagule development and senescence were used to calibrate the plant production and yield component. Total biomass estimates were within 5%, yield estimates were within 10%, and nitrogen uptake was within 20% of field measurements. Calibration of the carbon and nitrogen dynamics module produced results that were generally within 20-50kg ha-1 of measured values for soil profile NO-3-N. Independent evaluations of the calibrated model focused on total biomass, yield, N uptake, and nitrogen in the soil profile. Predictions were found to match the observed data in most cases. The crop model is very sensitive to plant N content. The model predicted biomass and yield well on irrigated and most dryland management systems and adequately simulated crop variables at various positions along the landscape.